[dev.typeparams] cmd/compile/internal/types2: print "incomplete" for interfaces in debug mode only

The /* incomplete */ comment printed for interfaces that have not been
"completed" yet is not useful for end-users; it's here for type-checker
debugging. Rather than trying to pass through a debug flag through all
print routines (which may require new exported API), simply don't print
the comment unless we have the debug flag set inside the type-checker.

For #46167.

Change-Id: Ibd22edfe63001dfd2b814eeb94c2d54d35afd88c
Reviewed-on: https://go-review.googlesource.com/c/go/+/320150


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>